1. Field of the Invention
This invention relates to radar tests devices and, more particularly, to radar test instruments which produce multiple replicas of a delayed radio frequency radar signal to simulate reflection of the radar signal off of distant targets.
2. Description of Related Art
In testing radar systems, it is desirable to simulate the reflection of a radar signal by a target located a great distance away from the radar detector. Simulations of the radar system are used because it is difficult to accurately point the radar at a test target and it is often difficult to prevent other objects from obstructing the radar signal from reaching the target thereby creating clutter. Other problems involved in such actual target tests include generating a signal having a high enough power to obtain a return signal from the distant target of adequate strength and coordinating the efforts between those individuals at the radar detector and those at the test target.
Because of the problems inherent in an actual distant-target test, radar test systems are used to simulate the effect of a distant target. A single coaxial cable can be used to accomplish this effect. However, because there are enormous losses at microwave frequencies in the signal strength associated with a long co-axial cable, the radar signal is usually coupled to an optical carrier. By utilizing an optical carrier traveling through fiber optic cable, low loss in the signal strength is obtained over long distances.
There are various ways to utilize optical delay means to simulate the distant target. First, an optical delay line equal in length to the distance to and from the distant target can be used. However, the cost and bulk of such a lengthy fiberoptic cable renders this approach prohibitive. A second approach is described in U.S. Pat. No. 4,903,029. Therein, the radar signal modulates a laser light which is controllably delayed in time by transmitting the modulated laser light through a fiber optic delay line. The output of the delay line is demodulated and the extracted and delayed radar signal is returned to the radar as a test signal. This delayed replica radar test set target system is only capable of producing a single delayed replica.
In order to calibrate a radar test instrument, it is often necessary to simulate different distances. In order to accomplish this, the radar test instrument must be capable of producing multiple echoes. U.S. Pat. No. 4,167,328 discloses a passive optical range simulator device for use with optical radar ranger systems. In this device, a partial mirror is included at the input end of an optical fiber coil to present multiple target echo reflections of the laser pulses back to the optical radar ranger system undergoing tests. The range simulator device of U.S. Pat. No. 4,167,328 is incapable of testing radar pulses, being limited solely to optical pulses. Moreover, the range simulator device of U.S. Pat. No. 4,167,328is not capable of sufficiently minimizing the loss in the system to provide consistently usable multiple echoes. Rather, the significant reflectivity losses occurring in the system cause each successive echo to achieve additional loss.